Electrostatic loudspeakers



Oct. 3, 1967 R. L. ROD 3,345,469

ELECTROSTATIC LOUDSPEAKERS I 3 Sheets-Sheet l Filed March 2, 1964 I ,3 IO f 4 INVENTOR 1' BY Roma.

ATTORN EYS Oct. 3, 1967 Filed March 2, 1964 R. L. ROD

ELECTROSTATIC LOUDSPEAKEBS 3 Sheets-Sheet 2 INVENTOR ROBERT L. ROD

ATTORN EYS Oct 1957 R. L. ROD 3,345,489

ELECTROSTATIC LOUDSPEAKERS Filed March 2, 1964 I 3 Sheets-Sheet 5 INVENTOR ROBERT L. RQD BVRCKULULQ ATTORNEYS United States PatentO 3,345,469 ELECTROSTATIC LOUDSPEAKERS Robert L. Rod, Pacific Palisades, Calif., assignor to Rod Development Corporation, Los Angeles, Calif a corporation of California Filed Mar. 2, 1964, Ser. No. 348,668 7 Claims. (Cl. 179-111) ABSTRACT OF THE DISCLOSURE An electrostatic loudspeaker comprised of a flexible wire mesh semi-rigid electrode and a flexible electrode with dielectric foil between them entrapping one or more strata of gas between one or more layers of foil and/ or the flexible electrode. The structure is flexible enough to be rolled into a cylinder'and used as a lampshade. The spacing between electrodes can be specified to provide bays of different spacings, to favor specified audible frequency bands.

This invention relates to loudspeakers and, particularly, to the class of sound reproducers operating on electrostatic principles. In such speakers, the forces required to move a sound-producing diaphragm are generated by varying electric charges applied toappropriate electrical conductors.

The objects of my invention are m'ultifold in number. One object is to make feasible the construction of electrostatic loudspeakers at acost equal to or less than moving coil loudspeakers of com-parable sound output levels and acoustical quality. Another object is to produce a full-range loudspeaker that faithfully reproduces the entire audio frequency spectrum up to about 20,000 c.p.s. Still another object is to build a loudspeaker that can be flexed or bent, as desired, for applications such as pillow and mattress speakers, collapsible talking movie projection screens, sound generating lamp shades, and others too numerous to list. Another important object of my invention is to simplify the construction of electrostatic loudspeakers by reducing the inordinately high signal and bias voltages heretofore required. In so doing, I have produced electrostatic loudspeakers that may be driven directly from the standardized 70.7 volt signal outputs of public address amplifiers and similar wired sound systems without intervening voltage step up transformers.

Despite their having certain points of superiority, electrostatic speakers of contemporary design have yet to supplant the universally used moving coil loudspeaker. The reasons for the poor acceptance of full range electrostatic loudspeakers developed up to the present time are many. They include undue mechanical complexity, a need for a comparatively large radiating area, and a dependence upon dangerously high voltages. Virtually all such electrostatic loudspeaker units (abbreviated hereinafter as ESLU for brevity) that have reached the market have made use of a taut, free-moving, electrically conductive diaphragm spaced in air some uniform distance away from one fixed electrically insulated rigid electrode or between two fixed insulated rigid electrodes. The essentially obsolete onefixed-electrode device is commonly called a singleended ESLU, while the more modern double-fixed-electrode unit is known as a push-pull speaker. To obtain adequate sound levels, particularly at lower frequencies,

without allowing the free-moving diaphragm to strike the fixed electrodes and so cause undesired distortion, wide spacings between elements have been employed. Wider spacings, in turn, have dictated a requirement for substantially higher drive and bias potentials. A typical fullrange push-pull ESLU now commercially offered to the public requires a bias source of 3,500 volts DC, and a driving amplifier with a power capacity of from to 75 Watts, the latter to achieve a sufiiciently high drive voltage across an almost purely reactive load. The unit has a radiating area of over six square feet on each face. To maintain the spacing between elements, a costly frame of rigid design and a multiplicity of auxiliary spacers is required to support the fixed electrodes and the taut diaphragm between them. These units often make use of elaborate segmentation techniques to provide uniform sound radiation patterns at various frequencies in the audible range. Clearly, such units offer little or no competitive advantages over moving coil electrod-ynamic speakers despite the intrinsic ability of their basic design to reproduce sound more faithfully.

While all the design considerations of an |ESLU are clearly beyond the scope of this application, it is useful to review the distinguishing features of a hypothetically ideal electrostatic speaker that could compete effectively with the moving coil speaker. These have been aptly stated by an eminent worker in the field, Dr. Frederick V. Hunt, head of the Harvard University Acoustics Laboratory, in his book, Electroacoustics (Wiley, 1954). For an ESLU to be capable of radiating energy over a usefully wide angle and of faithfully reproducing sound over audible frequencies ranging from below 100 cps. to 20,000 c.p.s., Hunt states (page 175) that there should be employed: (a) an extraordinarily light and strong diaphragm material; (b) a high polarizing voltage; (c) complete sheathing of the stationary electrode(s); (d) acoustic transparency of the stationary electrode(s); (e) subdivision of the diaphragm into elemental vibratory units, or bays; (f) systematic variation of the diaphragm stilfness by adjustment of the lateral separation; (g) systematic variation of the diaphragm separation from the backplate or stationary electrode(s); and, (h) variation in the diaphragm area with frequency.

Unless these various requirements are satisfied simultaneously, those familiar with the art will agree that it appears extremely difiicult to visualize development of an ESLU meeting the rigid performance standards of modern day reproducing systems. Unfortunately, meeting certain of these requirements has complicated the mechanical construction of existing electrostatic units to the point where they are not competitive with conventional moving coil. speakers. In my present invention, these requirements are met in a simple, straightforward manner, hereinafter described in detail.

According to my invention in one of its aspects, there is provided an electrostatic sound reproducer of the type having a movable electroded diaphragm and at least one fixed electrode adjacent thereto comprising, an acoustically transparent electrically-conductive semi-rigid fixed electrode, a flexible diaphragm comprising conductive electrode means extending over its active area-confronting said fixed electrode at one side thereof, dielectric means disposed between said electrode means and said fixed electrode, and means holding said diaphragm fixed 3 to said fixed electrode along a line perimetrically bounding said active area. The need for rigidity of the fixed electrode, heretofore imposed on this class of sound reproducers, is eliminated, with the result that loudspeakers according to my invention can be fixed or rolled, with new beneficial results to be described hereinafter,

In another of its aspects, my invention provides an electrostatic sound reproducer of the type having a movable electroded diaphragm and at least one fixed electrode adjacent thereto comprising, an acoustically-transparent electrically conductive fixed electrode, a flexible diaphragm comprising conductive electrode means extending over its active area confronting said fixed electrode at one side thereof, a dielectric film disposed between said diaphragm and said fixed electrode, means substantially hermetically sealing said diaphragm to said film along a line perimetrically bounding said active area whereby to entrap a stratum of gas between said diaphragm and said film, and means holding said diaphragm fixed to said fixed electrode along said line. The need for high bias voltage and high driving signal power heretofore imposed on this class of loudspeakers is eliminated and the inter-element spacing can be drastically reduced, all as described hereinafter in detail.

Moreover, these features can be combined to provide a thin, flexible electrostatic sound reproducer which can be operated at low bias voltage and driving signal power.

The foregoing and other objects and features of my invention will become more apparent from the following detailed description of certain exemplary embodiments of the invention. This description refers to the accompanying drawings, wherein:

FIG. 1 is a. vertical section through an electrostatic sound reproducer according to the invention, in which the horizontal scale is expanded or exaggerated to simplify the illustration;

FIG. 2 is a front view of a reproducer made according to FIG. 1 and having the additional feature that it is divided into bays or sections which individually favor different frequency ranges;

FIG. 3 is a partial vertical sectional illustrating the combination of my loudspeaker and a light-reflective acoustically-transparent material capable of serving the purposes of a picture-projection screen and a loudspeaker simultaneously;

FIG. 4 is a side view, partially in vertical section, of a lamp incorporating my loudspeaker as a lampshade;

' FIG. 5 is a cross-section taken along line 55 in FIG. 4;

FIG. 6 is a circuit diagram of the loudspeaker configuration shown in FIG. 4; and

FIG. 7 is a partial vertical section through another embodiment of my electrostatic sound reproducer.

In FIG. 1 is shown an elemental (e.g., one foot square) push-pull ESLU bay or unit as used in my invention, the horizontal scale being exaggerated to permit a clear illustration of some of its details. The exact size is not particularly critical and the exemplary size is cited only as an example. Neither is the shape or curvature particularly critical. In a sectional view vertically through the center of this typical bay, I show a centrally situated flexible dielectric diaphragm 1, preferably ranging in thickness from 0.25 mil to one mil, which has been coated on both faces with thin flexible electrically conductive layers 2, 3, preferably of evaporated aluminum or other lightweight material. The conductive layer can be limited to one side only, of the diaphragm, if desired. The diaphragm may be a metalized plastic film (as shown), or it may be a flexible film which is itself conductive (not shown). Polyethylene terephthalate, polypropylene or polyvinyl chloride, to name several commercially available materials, are in various degrees suitable as dielectric film materials for this application, since they are extraordinarily strong and lightweight. On either side of the diaphragm 1 are one or more thin plastic dielectric sheets 6, 7, 10, 11 (two pairs of two each being shown), which I call buffers for convenience. The buffers are fastened and sealed around their edges to both the diaphragm l and to each other by narrow bands of double backed pressure sensitive adhesive tape 18, 19, 20, 21. Alternatively, the peripheral seals may be made by heat sealing the sheets at the edges or with the use of appropriate adhesives. In any case, it is desirable that thin layers of air (or other gas) be trapped in the regions between the diaphragm 1 and the confronting buflers 6 and 7, and between the pairs of adjacent buffers, if two or more are used, as indicated at 4, 5, 8, 9. Dielectric buffer elements 6, 7, 10, 11 are preferably sheets of extremely thin, high dielectric strength film such as polyethylene terephthalate, which is commonly known by the Du Pont trade name, Mylar, or polyvinyl chloride, known by the Dow trade name, Saran.

Continuing the discussion of FIG. 1, which is drawn to an exaggerated horizontal scale for clarity, two socalled fixed electrodes 14, 15 are held a small but relatively uncritical distance away from the outermost buffers 10, 11, respectively, by means of peripheral spacers 22, 23, respectively. Average spacings of from 10 to 40 mils have been employed, but these are not limiting values. These spacers may be made of multiple layers of pressure sensitive adhesive tape or of any typical thin gasket sheeting, including compressed foamed plastic, cork, rubber, and the like. Electrical leads 16, 17 are connected to the electrically conductive fixed electrodes 14, 15, which are made of ordinary semi-rigid metal screening or wire mesh and which may be coated with electrical insulation materials (not shown) as will be described shortly. Another electrical lead 24 comprising the center-tap connection is made to the conductive diaphragm 1.

The entire structure shown in FIG. 1 might at first be thought to resemble the common push-pull ESLU that is old in the art. (See e.g., the United States patents to Vogt 1,881,107 or Kellogg 1,983,377.) Differences which account for the remarkable improvement in the performance of my invention over earlier electrostatic loudspeakers now will be discussed.

One of the most important features of my ESLU invention is use of a unique buffer arrangement in combination with semi-rigid fixed electrodes. As mentioned earlier, the contemporary push-pull ESLU invariably makes use of a sandwich arrangement in which a movable diaphragm with its integral conductive coating is suspended to move freely back and forth in the air spaces between two insulated and perforated rigid fixed electrode plates. Spacing is such that contact between the diaphragm and the plates is avoided, even at maximum sound volume. Otherwise, undesirable distortion would occur.

Under conditions of normal and elevated signal drive, additional distortion is also developed in contemporary ESLUs by the nonuniformity of the air layers in the regions bot-h near and afar from the perforations in the fixed electrodes that are provided to allow the sound to escape. Ideally, the fixed electrodes should be acoustically transparent, but this is not always the case with contemporary units. In my invention, t'he effects of this nonuniformity have been substantially reduced by my use of semi-rigid mesh electrodes 14, 15 which are preferred because they have a greater ratio of open area to supporting structure than that possible of achievement with perforated metal plates. In fact, less critical applications from the standpoint of fidelity can be accommodated without using any buffers at all providing the fixed mesh electrodes 14 and 15 are adequately insulated, preferably with a high dielectric constant material. In an alternative method also suitable for less critical applications, the buffers may be omitted by using a pair of thin plastic diaphragms coated with conductive layers on their facing surfaces only, as is illustrated in FIG. 7. The fixed electrodes 14 and 15, which are similar to the corresponding electrodes in FIG. 1, have a flexible diaphragm 51 between them, which comprises two con-fronting sheets 52, 53 of dielectric material having thin flexible electrically conductive electrode layers 54, 55, respectively, on their confronting surfaces. The opposite faces are then adequately insulated in themselves, but additional insulation of the fixed electrodes 14, 15 is a well-taken procedure.

In most applications, however, the ultimate in quality is desired at relatively high sound pressure levels. Thus, I have found that by using buffers 6, 7, 10, 11 great improvements in performance result above that realized with my semi-rigid mesh electrodes and no buffers. For one thing, the use of at least one buffer between the vibrating diaphragm and each fixed electrode makes the intervening air layer more uniform. The air layers between the diaphragm and the buffer and between the buffer and the fixed electrode on each side of the unit may then be considered together. It can be shown mathematically that the thickness of the combined air layers is more uniform than a single air layer without buffers and that the arrangement is less prone to the creation of distortion. In practice, more uniform air layers permit greater sound levels to be developed on relatively smaller diaphragm excursions than heretofore required. Thus, smaller spacings between the diaphragm and the fixed electrode either side of it can be used to advantage, and it becomes feasible to build these units in the simple manner described.

The use of a multilayer system is best accomplished by taking steps to control the various trapped air films. If no pains are taken to seal the edges as described previously, a noisy pumping-section action will be observed as the various foils move with respect to each other. By sealing the edges, this annoyance is eliminated. In the design of ESLUs as described herein variations are permissible. As mentioned above, the buffers may be omitted in less critical applications, since my design encompassing wire mesh fixed electrodes provides a significantly more uniform air layer than that encountered heretofore with units made with perforated metallic fixed electrodes. On the other hand, even finer performance may be achieved by using two or more buffers on each side of the speaker diaphragm 1, so long as the total thickness of the various films is kept at a reasonable -minimum. Alternatively, single-ended ESLUs may be fabricated using the techniques outlined herein although, as is well-known, their ultimate performance capability is generally unsatisfactory in terms of distortion-free output. While I do not Wish to be limited by any technical explanation of the operation of my invention, I believe that some of its advantages may be explained as follows.

Still another advantage. inherent in the use of bulfers 6, 7, 10, 11 involves improvements in the magnitude of the electric charge over that achieved in a contemporary ESLU using air alone as the dielectric between the electrodes. Conventional film materials I have used as buffers have a dielectric constant several times that of air. By placing such materials between the diaphragm and the respective fixed electrodes, the electric charge created by the bias and signal voltages that exists between electrodes is increased by a factor which bears a relation to the ratio of the dielectric constants of the buffer material employed and air. The net effect of this improvement is that the combination of signal and bias voltages required to move the diaphragm a certain arbitrary distance has been reduced over that required in a conventional ESLU of similar dimensions. The resulting development of more uniform air layers and increased electric charge renders it possible to build an inexpensive electrostatic loudspeaker unit far thinner and more flexible than units of the type heretofore available, without in the uniformity of the field itself. In an ESLU using air alone as the dielectric between conventional electrodes, there is a finite limit to the degree of acoustical transparency that can be achieved with practical fixed electrodes. In the case of mesh fixed electrodes, the conductors must be reasonably close to each other to insure creation of uniform electric fields. Otherwise, uniform diaphragm motion would be achieved only in segments close to conductive surfaces comprising the fixed electrode. This requirement for greater conductor surface tends to reduce the acoustical transparency of the fixed electrode in an adverse manner. For a given mesh size or opening in the fixed electrode, greater field uniformity is achieved in my invention by using the higher dielectric constant material I refer to as the buffers 6, 7, 10, 11. This is accomplished without reducing the elfectivity of the signal and bias voltages required and without restricting the passage of sound through the fixed electrode. Even further improvements in the field strength and uniformity can be achieved by coating the fixed electrodes with a high dielectric constant insulating material, a technique ascribed to Vogt 1,881,107.

Before turning to more elaborate versions of my invention, it is useful to elaborate upon my earlier statement that the thickness of the buffers 6, 7, 10, 11, or combinations thereof, should be kept to a minimum, preferably 0.25 mil each and no greater than several mils in total. Two reasons for this become apparent. The most obvious is that a buffer should be of light mass and have an acoustical impedance as close to air as possible. Otherwise, severe sound damping and loss in efliciency will become evident. The thinnest films thus are most useful.

Another feature of my invention is that the need for rigid fixed electrodes of perforated metal and the complex mounting frame has been obviated by my use of selfsupporting, semi-rigid metal screening or wire mesh electrodes 14, 15 that requires no special mounting provisions. I have found that this type of material forms an ideal fixed electrode because, (a) it is almost perfectly transparent acoustically Whereas perforated metallic plates are not; (b) it is acoustically dead and thus does not rattle or self-resonate on sound peaks at certain frequencies; (c) it is sufiiciently rigid intrinsically, along with the support given by the opposite fixed electrode in the push pull case, to hold the over-all bay in shape; and, (d) it can, but need not be electrically insulated, as desired, with sprayed, brushed or dipped dielectric coatings, or by anodizing in the case of aluminum wire screening. In most applications, sheathing the fixed electrodes with high dielectric constant coatings is desirable for further increasing the strength and uniformity of the electricfield and for protecting users against contact with electrical conductors.

The use of wire screens as the unsupported fixed electrodes is novel over Kyle United States Patent No. 1,644,387, which uses Wire mesh merely for mechanically separating elements in a single-ended ESLU. Klar in United States Patent No. 1,813,855 shows an alternative embodiment of his single-ended ESLU which uses a wire screen electrode held taut in a strong circumferential ring. An important feature of my invention is that by deliberately not supporting the wire screen electrodes rigidly, a very desirable differential spacing is created naturally between the screens 14, 15, the buffers 6, 7, 10, 11 and the diaphragm 1. These differential spacings very simply satisfy Hunts stated requirement for a systematic variation in the spacing between the diaphragm and the fixed electrode(s), a requirement that should be satisfied if sound output from a flat sheet ESLU isv to be uniform in magnitude and reasonably broad in directionality over the working frequency range. Moreover, my invention makes it possible to flex 'or roll the entire loudspeaker unit, since the semirigid fixed electrodes 14, 15 are made of a material (e.g. metal screening or wire mesh) which is readily flexed by hand.

Another feature of my invention is that the diaphragm 1 is not held particularly taut in a complex and costly frame as in earlier loudspeakers of this type. The over-all rigidity of a bay made in accordance with my principles is such that except for regions near its clamped edges, the diaphragm stiffness is a variable. This feature satisfies another of Hunts requirements, namely a systematic variation in the diaphragm stiffness. Such variation tends to further improve the uniformity of the frequency response.

In FIG. 2 is shown a typical ESLU of my design having approximate dimensions of 3 feet by 1 /2 feet by less than /2 inch in overall thickness. Such a unit has an adequate radiating area to produce sound levels of the magnitude required in general home entertainment use and in offices, restaurants and other public places. In this figure, I have illustrated the mechanical construction details of a multi-bay unit, employing the same reference characters as in FIG. 1 for like elements. In the upper right hand corner, I have, for illustrative purposes, shown peeled off the various elements comprising the unit. There can be seen the fixed wire mesh electrodes 14, buffers 6, 7, 10, 11; central diaphragm 1 and its conductive coatings 2, 3; buffer-fixed electrode spacer 22 and an added edging frame 25 to hold the complete assembly together. For clarity in illustration, I have omitted from the drawing spacer 23 and the means for sealing the edges of the buffers and the diaphragm as previously described.

The major difference between the elemental bay shown in FIG. 1 and the multi-bay unit shown in FIG. 2 is the use of cross-members 30, 31, 32 (shown in solid-lines) to subdivide one larger unit into a number of smaller bays, in this case indicated at 26, 27, 28, 29. The crossmembers are of identical construction to those used in FIG. 1 to seal and separate the various buffered films at their edges. In effect, four bays have been made from one, each bay being sealed from the others. However, fixed electrodes 14, 15; diaphragm 1 and the buffers 6, 7, 10, 11 are all of identical size equal to the overall dimensions of the unit.

My use of a larger unit insures generation of sound at usefully high levels. It further permits me to make slight modifications to the frequency response by altering electrode spacings within the individual bays. Such a step is desirable in achieving a uniform distribution of various sound frequencies throughout the room in which the ESLU is used. As is well known, a large radiator of high frequency sounds will tend to focus the energy in a narrow beam. In the case of a flat ESLU that makes no allowances for this, low frequency sounds are broadly radiated, but the higher frequencies are concentrated in planes perpendicular to the diaphragm. The larger the unit, the more concentrated are the higher frequency sounds. In my invention, I overcome a problem especially noticeable in a fiat ESLU by varying the overall spacing between the diaphragm 1 and the fixed wire mesh electrodes 14, 15 in a predetermined manner. As a typical example, in bay 26, spacers 22, 23 are thinner than those employed in the remaining bays 27, 28, 29. With less spacing, higher frequency sounds are. radiated more efficiently in bay 26 than in the other three. Thus, with a smaller radiating area for the higher frequencies being employed than that made available for the lower frequencies, the resultant higher frequency sounds are less focused. On the other hand, bays 27, 28, 29 are more efficient radiators of low and medium range frequencies because of the added spacings being provided between the diaphragm and the two fixed electrodes. These allow for greater diaphragm excursions.

In examining the units illustrated in FIGS. 1 and 2, it should be pointed out that a large number of variations in the size and form factor may be employed without changing the principles inherent in my invention. For one thing, the flat sheet-like structure can be bent, or flexed, to

achieve a more desirable radiation pattern or to conform with the physical environment in which it may be located. Larger units can be made with fewer or more bays to achieve greater sound levels and varied response over certain desired frequency ranges. With proper choice of materials, the ESLU can be made extremely flexible so that 'it can be rolled up like a window shade. In such a case, an outer face of one of the fixed wire mesh electrodes 15 can be provided with a suitable acoustically transparent cloth 15.5 to serve a combined motion picture projector screen and loudspeaker cover, as is shown in FIG. 3. Similarly, outer coverings that are acoustically transparent can be employed for decorative or advertising purposes. on such units.

Another use of an ESLU that is made possible with my invention is in a lampshade. In most homes, present high fidelity entertainment systems make use of loudspeakers requiring excessive space. It is also difficult to blend these speakers and their enclosures into the decorating motif. By combining the functions of a lamp and a loudspeaker, it is possible to substitute my ESLU in circular form for a conventional lampshade and to so obtain uniform sound distribution throughout a room. No particular pains need be taken to balance the radiation patterns obtained from this ESLU at various frequencies. In circular form, all frequencies are radiated uniformly in the horizontal plane. Since the usual lampshade is almost at ear level, radiation in the horizontal plane from my ESLU appears equal at various frequencies and is thus very satisfying aesthetically.

In FIGS. 4 and 5 is shown a very simplified sketch of my ESLU as applied to a lamp assembly of the type commonly found in the home. FIG. 5 is drawn to larger scale than FIG. 4, to facilitate illustration of its details. ESLU 33 is mounted as a lampshade in place of the conventional shade on the same mounting framework ordinarily used. Elements similar in function to elements in FIG. 1 bear the same reference characters. The speaker is a circular version of the elemental bay I described in FIG. 1, and only the outer fixed mesh electrode is illustrated 15 in FIG. 4. For decorative purposes, the shape preferably is covered on the inner and outer surfaces by usual lampshade materials (not shown) that may be held in place by edging tapes 40*, 40. These lampshade materials, of course, should be and frequently are acoustically transparent. Means (not shown) for supporting the ESLU 33 as a lampshade can be provided in a known manner.

In the base of the lamp are shown the usual electrical components required with an ESLU, namely the bias power supply and input signal transformer, all enclosed in a protective box 35. Conductive leads 34 feed bias and signal to the lampshade, while power cable 39 feeds mains voltage to the lamp and to the bias supply. Also shown in FIG. 4 is an optional loudspeaker 37 of the conventional electrodynamic type which may be used to augment the production of very low frequency sounds. This speaker is driven simultaneously with the ESLU and is arranged to reproduce only the lowest frequency sounds by means of a conventional cross-over network incorporated within assembly 35 or by virtue of its own construction. Cable 42 connects the sound generating portions of the unit to the radio or phonograph amplifier located elsewhere. Although the ESLU is capable of faithful reproduction of lower frequency sounds, the general public often has a preference for hearing these boosted in amplitude above a natural level. For this reason, I frequently employe the optional woofer speaker 37 to augment sound output over this portion of the sound spectrum when and if it is needed or desired. Base 38 is employed to support the entire structure, and to direct the output of the speaker 37 omnidirectionally throughout the room by means of a circular exponential horn comprised of the top surface 43. It will be noted that speaker 37 is arranged for convenience to radiate downwards, but other mounting positions can be employed if desired.

In considering my ESLU as a lampshade-speaker, two minor but preferable modifications in construction are worthy of mention, since both are optional and do not affect my invention. The first involves the use of very thin conductive layers 2, 3 on the diaphragm 1 which then become light transparent rather than opaque, as is 1181.1211- ly the case. The reason for this is to allow some light to penetrate the shade in operation and further simulate an ordinary lampshade. The second, .as shown in FIG. 5, is a simplification in the design involving wrapping the buffer material 610 and 711' around the shade two or three more times until the desired number of buffer layers are arrived at. This approach eliminates the need for a number of separate buffer films, each concentric to the others, and so simplifies the construction. Obviously, neither these changes nor changes in size and shape will affect the basic principles underlying my invention.

In FIG. 6 is shown a functional schematic diagram of one circuit I have used to energize the ESLU 33 shown in FIG. 4. Leads 42 comprise the electrical signal input from a conventional amplifier having low output impedance. Since an ESLU is esentially a high impedance reactive load and requires a relatively high driving signal voltage, I employ in this case the usual step-up transformer 46 to develop the signal input voltages for ESLU 33 from signals developed across the low impedance terminals of conventional amplifiers. These signals are applied to the fixed electrodes 14, 15 through conductors 16, 17 about a center tap connection 47 in the secondary of transformer 46. Necessary bias voltage is developed by battery 45. In series with the bias voltage developed by battery 45 is a resistor 44 of very high resistance value which serves a dual purpose of limiting current flow should short circuits develop in the ESLU and of minimizing distortion otherwise developed by the speaker. The use of this resistor for limiting distortion has evolved from the work of Professor Hunt and is described in his aforementioned text.

Also shown in FIG. 6 is optional woofer speaker 37 energized via conductors 41. This speaker may be especially built to emphasize low frequencies or it may contain a built-in crossover network of conventional design.

In FIG. 6, it should be noted that many variations of the circuit are permissible. Transformer 46 may be eliminated in installations where adequate signal voltages are provided. Battery 45 can be replaced by a conventional rectifier-type power supply energized from the mains voltage. Modifications can be made in the method of applying the signal voltages to the ESLU; such changes do not affect my invention, which concerns itself with my new ESLU per se.

It will be understood that the semi-rigid fixed electrodes 14, 15 of my invention can be made of wire mesh or screen, as shown, or, if desired, of an array of wires strung across the reproducer, or of any other suitable configuration of conductors. In embodiments including the buffers 6, 7, 10, 11, or any pair of these, reproducers according to my invention may employ rigid fixed electrodes according to the prior art, without loss of the low-voltage features of the invention.

The embodiments of the invention which have been illustrated and described herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. For example, materials suitable for use as the diaphragm include fiber glass cloth, woven synthetic cloths, and paper, all of which have been treated to close their pores and made electrically conductive on one or both sides, in addition to the materials mentioned hereinabove. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also within the spirit and scope of the invention, and it is intended that this invention includes all 1t) modifications and equivalents which fall within the scope of the appended claims.

What is claimed is: 1. An electrostatic sound reproducer of the type having a movable electroded diaphragm and at least one fixed electrode adjacent thereto comprising, an acousticallytransparent electrically conductive fixed electrode made of a semi-rigid material capable of being flexed by hand into a desired shape, a flexible diaphragm comprising conductive electrode means extending over its active area confronting said fixed electrode at one side thereof, a dielectric film disposed between said diaphragm and said fixed electrode, means substantially hermetically sealing said diaphragm to said film along a line perimetrically bounding said active area whereby to entrap a stratum of gas between said diaphragm and said film, and means holding said diaphragm fixed to said fixed electrode along said line.

2. An electrostatic sound reproducer of the type having a movable electroded diaphragm and at least one fixed electrode adjacent thereto comprising, a metallic wire mesh fixed electrode, a flexible dielectric diaphragm confronting said fixed electrode at one side of said diaphragm, a dielectric film disposed between said diaphragm and said fixed electrode, means substantially hermetically sealing said diaphragm to said film along a line perirnetrically bounding the active area of said diaphragm whereby to entrap a stratum of gas between said diaphragm and said film, means holding said diaphragm fixed to said fixed electrode along said line, and a flexible electrically conductive electrode aflixed to the other side of said diaphragm over said active area.

3. An electrostatic sound reproducer according to claim Zhaving a second electrically conductive electrode aflixed to said one side of said diaphragm over said active area.

4. An electrostatic sound reproducer according to claim 3 comprising a second metallic wire mesh fixed electrode confronting the other side of said diaphragm, a second dielectric film disposed between said diaphragm and said second fixed electrode, means substantially hermetically sealing said diaphragm to said film along said line whereby to trap a second stratum of gas between said diaphragm and said second film, and means holding said diaphragm fixed to said second fixed electrode along said line.

5. An electrostatic sound reproducer comprising a semi-rigid metallic wire mesh fixed electrode, a dielectric confronting the dielectric film at one side and supported at its edges to the dielectric film and sealed whereby to entrap a stratum of gas between the film and the diaphragm, and a flexible electrically conductive electrode afiixed to the opposite side of the diaphragm.

6. An electrostatic sound reproducer comprising two metallic substantially concentric cylindrical semi-rigid wire mesh fixed electrodes, at least one continuously wound spiral forming a plurality of layers of a dielectric foil sealed at the spiral edges thereof to each of the fixed electrodes to form at least one entrapped stratum of gas therein, said spirals located between said electrodes, a flexible diaphragm located between said spirals of the dielectric foil, and means to entrap a stratum of gas between each of said spirals and said diaphragm, flexible electrically conductive electrode means afiixed to at least one side of the flexible diaphragm, and means to hold the spiral and cylindrical end edges of the complete struc ture in close contact.

7. An electrostatic sound reproducer comprising two metallic substantially concentric cylindrical semi-rigid wire mesh fixed electrodes, at least two assemblies of layers of dielectric foil sealed at their edges to form a plurality of entrapped gas strata therebetween, one located next to each of the fixed electrodes between said fixed electrodes and concentric therewith, two flexible diaphragms located between said foil assemblies with their confronting surfaces covered with flexible electrically con- References Cited UNITED STATES PATENTS 7/1929 De Forest 325---31O 4/ 1942 Crowley 325-3 10 1 2 Curry 179-111 Lindenberg 179-41 1 Stanton 179-111 Gorike 179111 Wright 179--111 Brettel 179-1 11 KATHLEEN H. CLAFFY, Primary Examiner.

R. P. TAYLOR, Assistant Examiner. 

5. AN ELECTROSTATIC SOUND REPRODUCER COMPRISING A SEMI-RIGID METALLIC WIRE MESH FIXED ELECTRODE, A DIELECTRIC FILM CONFRONTING THE FIXED ELECTRODE, A FLEXIBLE DIAPHRAGM CONFRONTING THE DIELELECTRIC FILM AT ONE SIDE AND SUPPORTED AT ITS EDGES TO THE DIELECTRIC FILM AND SEALED WHEREBY TO 